Wind Turbine Foundation Behavior and Design Considerations
The paper was presented at the AWEA WINDPOWER Conference in Houston earlier in June. Download the paper and/or read the abstract below.
Abstract: Despite certain aspects of wind turbine foundations which are common industry-wide, there is marked diversity in the understanding of the foundation behavior and application of design codes. This has led to different foundation designs for identical turbines with similar geotechnical conditions. The wind industry continues to grow using larger multimegawatt turbines. An understanding of the driving considerations for the associated growth of the foundations will permit safe and economical designs while giving due consideration to design codes that may not explicitly address wind turbine foundations. This paper discusses the key design criteria and components inherent to wind turbine foundation design, specifically the spread footing. Potential impacts to these areas from larger turbines and how the engineering/construction community may address them are explored.
The paper focuses on spread foundations for on-shore wind turbines, so I asked Eric what percentage of foundations were spread foundations vs. deep foundations. He estimated that about 70% or more of foundations were spread foundations. He also indicated that in some areas with poor soil conditions, micropiles were one form of foundation that has been used in the past. He didn’t say if they ever use large diameter drilled shafts, maybe they aren’t economical enough for this application.
Eric also gave me some typical dimensions to satisfy my own curiosity. He said that the average footing size for these foundations is about 50-60 ft in diameter, 5-ft thick and they are usually octagonal in shape. He was kind enough to furnish a picture of one footing. he said this one was 50-ft in diameter and 8-ft thick at the pedestal. (Photo courtesy of Eric Ntambakwa, Garrad Hassan America, Inc.)
Monstrous Off-shore Wind Turbines in Belgium
ENR reported on "Big-Bladed Wind Beasts Need Behemoth Foundations". These turbines are 5MW models. I don’t know this for certain, but I think typical on-shore wind turbines you see on wind farms are 1.5 to 3 MW. The water in the North Sea where these units are being installed is about 27-m deep and the blades themselves have a diameter of 126-m! The foundation system is a single massive hollow shaft over a tapered "cone". They look like belled drilled shaft foundations. They are created on-shore and barged to the site and sunk into a dredged trench on the sea floor and backfilled overe. Then they are filled with sand ballast. (Photo by C-Power N.V. by way of ENR)